I had a feeling it would be something like this. Kyouko and Naachan were in love, Naachan became jealous and scared so she decided to abandon her relationship with Kyouko, but ended up regretting it in the end. Honestly I'm a little upset now, like @TurboTime said, I seriously doubt there'd even the slightest bit of chance of them getting back together after she finds out Kyouko's been messing around with her little sister.